One of the most important challenges in horizontal well fracturing is to create multiple tightly-spaced hydraulic fractures (HFs) within each fracturing stage in tight reservoirs, especially with the strong heterogeneity. This study explores the influences of static fatigue and pressure shock on the simultaneous initiation of multi-HF from an open-hole horizontal wellbore in a formation with the variability in the rock strength. Laboratory-scale fracturing experiments were conducted on cuboid-shaped specimens (8 cm × 8 cm × 10 cm) of varying brittleness under the tri-axial confining stress. The rock strength variability in a specimen was illustrated as the variation in the width or length of the preset natural fractures (NFs) within the open-hole section (OHS). Inside the wellbore, three pressurization regimes, corresponding to the normal, static fatigue and pressure shock conditions, were obtained by the constant-rate injection, constant-pressure injection, and high-pressure build-up and rapid release, respectively. The results demonstrate that under the pressure shock conditions, bottom hole pressure (BHP) can be forced to exceed the normal breakdown pressure in a conventional fracturing, and generate a relatively high breakdown pressure. It has the potential to improve the simultaneous initiation of multiple transverse HFs from NFs of different strengths and that of longitudinal HF within a single fracturing stage. A lower limit BHP is required to overcome the variations in strength across different potential initiation points. Meanwhile, the low-BHP, which is lower than the normal breakdown pressure, is maintained to induce the static fatigue failure of the surrounding rock. Under static fatigue conditions, a single HF is generally initiated from the weak NF. The time to breakdown decreases evidently with the increase in the BHP and rock brittleness. In low-brittle specimens, the time-dependent initiation of HF may be difficult in the OHS without connecting with the NFs. In practice, simultaneous initiation of multi-HF may be enhanced by the combined effects of static fatigue and pressure shock.

中文翻译：

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静疲劳压力冲击驱动多条水力裂缝同时起裂的试验研究

水平井压裂最重要的挑战之一是在致密储层的每个压裂阶段内创建多个紧密间隔的水力裂缝 (HF)，尤其是具有强非均质性的油藏。本研究探讨了静态疲劳和压力冲击对岩石强度变化的地层中裸眼水平井眼同时起爆多 HF 的影响。实验室规模的压裂实验是在三轴围压下对不同脆性的长方体形试样（8 cm × 8 cm × 10 cm）进行的。样本中岩石强度的变化被说明为裸眼段 (OHS) 内预设天然裂缝 (NF) 的宽度或长度的变化。在井筒内，对应于正常的三种加压方式，静态疲劳和压力冲击条件分别通过恒速注射、恒压注射和高压建立和快速释放获得。结果表明，在压力冲击条件下，可以迫使井底压力（BHP）超过常规压裂的正常破裂压力，并产生较高的破裂压力。它有可能在单个压裂阶段改善从不同强度的 NF 和纵向 HF 同时启动多个横向 HF。需要一个下限 BHP 来克服不同潜在起始点的强度变化。同时，维持低于正常破坏压力的低BHP，诱发围岩静疲劳破坏。在静态疲劳条件下，单个 HF 通常从弱 NF 开始。随着BHP和岩石脆性的增加，击穿时间明显缩短。在低脆性试样中，如果不与 NF 连接，在 OHS 中可能难以随时间启动 HF。在实践中，静态疲劳和压力冲击的联合作用可以增强多高频的同时启动。